1. Field of the Invention
This invention relates to position sensors. In particular, there is a carbon fiber wiper for use in a position sensor. The wiper has a long life.
2. Description of the Related Art
Various devices and methods of dealing with the design of position sensors are legion. More specifically, contacting position sensors have been formed from a contactor tip and a resistive element. The tip moves over the surface of the element and causes an output reading to change. Polymer resistive position sensors are a variable resistor type of electrical devices with a voltage output that converts the linear or angular displacement to an electrical signal. The contactors are conductive moving elements in the sensors that control the voltage output by sliding over a polymer resistive element. These contactors represent a unique type of sliding electrical contacts. They are miniature in design and size. The contactor is typically an alloy of platinum, silver, palladium and gold. The current density is low at approximately 10 A/sqin. or less. They run at slow sliding speeds, around 50 fpm or slower. The contactor should be capable of making good electrical contact under all the application environments, basically, heat and cold, dry and humid, dust, vibration, and etc as other types of contacts do. However, the reliability and noise performance requirements are more crucial than with other types of sliding contacts. The contactor must also have good corrosion resistance.
Currently, the contactors of this type are constructed with metallic materials or alloys, because they are good conductors, easy to process and readily available. Noble alloys such as gold, palladium and platinum alloys are often used because of their non-tarnishing character in all the common air pollutants. These noble alloy contactors have been used for decades, and have been well accepted in the industry. Because of their industry-wide importance the American Society for Testing and Materials has adopted numerous standards for these alloys, as shown in ASTM Standards Vol. 03.04.
As the contactor tip moves against the element surface wear occurs on both surfaces. The prior art contactor tips are formed from a metal having good wear resistance and spring characteristics, for example beryllium copper. The prior art resistive elements have been formed from carbon black and polymer combinations that are screened onto a substrate and dried and cured.
Examples of patents related to the present invention are as follows, and each patent is herein incorporated by reference for the supporting teachings:
U.S. Pat. No. 4,336,565 is a charge process with a carbon fiber brush electrode.
U.S. Pat. No. 4,568,876 is a method of testing a wiper of an electric potentiometer.
U.S. Pat. No. 5,139,862 is a pultruded electronic device.
U.S. Pat. No. 5,177,529 is a machine with removable unit having two element electrical connection.
U.S. Pat. No. 5,843,567 is an electrical component containing magnetic to particles.
U.S. Pat. No. 3,886,386 is a carbon fiber current collection brush.
U.S. Pat. No. 4,189,702 is a commutator and fiber brush rotating disc.
U.S. Pat. No. 4,345,235 is a variable resistance device.
U.S. Pat. No. 5,725,707 is an enhanced conductive joint.
U.S. Pat. No. 4,358,699 is a versatile electrical fiber brush.
U.S. Pat. No. 4,761,709 is a contact brush charging.
U.S. Pat. No. 4,553,191 is a static eliminator.
The foregoing patents reflect the state of the art of which the applicant is aware and are tendered with the view toward discharging applicants"" acknowledged duty of candor in disclosing information that may be pertinent in the examination of this application. It is respectfully stipulated, however, that none of these patents teach or render obvious, singly or when considered in combination, the applicant""s claimed invention.
3. Problem with the Related Art
There are several common problems occurring with the prior art. As the metal contactor tip moves against the element over time, it can gouge and pit the element causing the resistance reading to fail or to generate an erroneous resistance reading. Another problem with the prior art devices is that over time the element material can be removed from the element and build up as a loose layer over the surface of the element. When the contactor tip is moved it skates across the surface of the loose material instead of making electrical contact with the element. This causes the resistance reading from the element to appear as an open circuit with infinite resistance and essentially causes the sensor to be inoperative for a period of time until contact is reestablished by the contactor tip to the element.
Poor electrical contacts have often been found associated with the metal contactors, even with noble alloy contactors. As a result, electrical noise is generated from these contactors, which lowers the performance and reliability, and ultimately causes failure of the sensor devices. Excessive wear either from the contactor itself or from the polymer resistive elements has also often been found in this type of sensors, which reduces the durability and even causes premature failure of the sensor devices.
Prior art contacting position sensor designers have attempted to solve these problems by introducing a lubricant into the contactor tip/element interface. A typical lubricant is an emulsification of Teflon particles. The lubricant reduces the frictional forces between the wear surface while still allowing electrical contact to be made between the contactor tip and the resistive element. The liquid lubricant is applied to the contact surfaces to reduce the wear. However, most of the liquid lubricants are poor electrical conductors, thereby causing relatively high electrical resistance across contact surfaces and possible failure of the devices where lubricants have been used improperly. The viscosity of the liquid lubricant changes significantly over the application temperature range, resulting in poor contact at low temperatures and insufficient lubrication at high temperatures. There are several other problems that arise from using a lubricant in the contactor system. First, the amount of lubricant applied is critical to the performance of the system. If too little lubricant is applied, the system will have excessive wear. If too much is applied, erroneous resistance readings due to skating will occur. It is very difficult to repeatably apply a uniform layer of the lubricant. Second, using a lubricant adds additional cost in material and labor to the overall sensor cost.
There are several common problems occurring with the previous arrangements of electrical fibers. The previous arrangements of electrical fibers have been primarily concerned with the removal or application of a static charge. These arrangements do not have the requirements of precise alignment and motion control that is required in a position sensor. Also, the previous arrangements allow the fibers to flex excessively. This would cause a large error in output readings if they were used in a position sensor. The previous arrangements have also been difficult to repeatably manufacture.
Therefore, there is a need for a less expensive and easily manufacturable electrical wiper for use in position sensors.
It is a feature of the invention to provide an electrical wiper for use in a position sensor to contact an electrically resistive surface. In particular, the wiper includes a metallic beam having a channel and a first and second substantially parallel extending flanges forming the channel. The first flange is attached to the beam. A connecting flange is connected to the first and second flanges. Several carbon fibers having a proxil end are fixedly secured within the channel between the first and second flanges and a distal end extends outwardly from the channel. The flanges have several tabs for securing the fibers in the channel. The tabs can be welded to the flanges.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. Further, the abstract is neither intended to define the invention of the application, which is measured by the claims, neither is it intended to be limiting as to the scope of the invention in any way.